MOVs are widely used as electronic components for suppressing surge voltages generated commonly in conventional electronic devices, such as surge absorbers, PTC (positive temperature coefficient) thermistors, NTC (negative temperature coefficient) thermistors and ceramic capacitors. Generally, a typical MOV has non-linear resistance properties in which resistance is high at normal power voltage but disproportionately low at high voltage exceeding predetermined levels.
As is well known, if the MOV has been used for a long time, the original high impedance of the resistance could become low, and a leakage current relative to the MOV may happen. In some worse cases, the leakage state will deteriorate gradually and thus the leak current may finally form a leak point with material thereof being melted and producing a short circuit point of about 1K Ohm. Then, if power source continues to impulse a substantial strong current into the short circuit point, the MOV will be caused to be overheated and caught fire. In addition, thermal energy results from instantaneous over-voltages due to lightning strike, switching of power or the like will make the MOV an increase in the temperature, which also unadvantagely leads to resistance punch or combustion.
One way used to protect the MOV at present is introduction of thermal cut-off fuse which is wired with the MOV and positioned adjacent one face of the MOV. The melting point of the thermal cut-off fuse is at a temperature below what is required to put the MOV in thermal runaway. As the temperature at the face of the MOV rises, a point is reached at which the thermal cut-off fuse melts and opens one lead to the MOV which no longer receives current. However, as it is epoxy and air that serve as the heat transportation medium, and as the epoxy has a limited thermal conductivity and the surrounding air could cool down or substantially influence the heat being transferred to the thermal cut-off fuse, sometimes thermal fuse cuts off the power just after the MOV becomes failed and burns, thus the prior approach can not prevent combustion of MOV efficiently.
Hence, a need has arisen for providing an improved MOV having thermal cut-off function to overcome the above-mentioned disadvantages.